1. Field of the Invention
The present invention relates to an ink jet recording method for recording an image by ejecting ink droplets onto a recording medium.
2. Related Background Art
With spread of copying machines, information equipment such as wordprocessors, computers, and the like, and communication equipment, an apparatus for performing digital image recording using an ink jet recording head has become increasingly popular as one of recording apparatuses for such equipment. A recording apparatus of this type uses a head prepared by integrating a plurality of ink ejection orifices and ink channels as a recording head (to be referred to as a multi-head hereinafter) in which a plurality of recording elements are integrated and aligned for the purpose of an increase in recording speed. Furthermore, a color recording apparatus normally comprises a plurality of multi-heads.
Unlike in a monochrome printer for printing characters alone, when a color image is to be printed, various characteristics such as color development characteristics, gradation characteristics, uniformity, and the like are required. In particular, as for the uniformity, a small variation in units of nozzles, which is generated during a multi-head manufacturing process, influences the ejection amount or ejection direction of an ink of each nozzle, and consequently deteriorates image quality as a density nonuniformity of a printed image.
An example of the density nonuniformity will be explained below with reference to FIGS. 12A to 13C. In FIG. 12A, a multi-head 91 is constituted by eight multi-nozzles 92 for ejecting ink droplets 93. Normally, the multi-nozzles 92 ideally eject the ink droplets in a uniform amount and in a uniform direction, as shown in FIG. 12A. If such ejection is performed, dots having a uniform size land on a sheet surface, as shown in FIG. 12B, and a uniform image free from the density nonuniformity can be obtained as a whole (FIG. 12C).
However, in practice, each nozzle suffers from a variation, as described above. If a print operation is performed in the same manner as described above, ink droplets having various sizes are ejected from the nozzles in various directions, as shown in FIG. 13A, and land on a sheet surface, as shown in FIG. 13B. As shown in FIG. 13B, a blank portion, which does not satisfy an area factor of 100%, conversely, a portion where dots unnecessarily overlap each other, and a white line (at the center of FIG. 13B) periodically appear in the head main scan direction. The group of dots which landed in the state shown in FIG. 13B has a density distribution shown in FIG. 13C in the nozzle alignment direction, and consequently, such a phenomenon is normally observed as a density nonuniformity by the human eye.
As a countermeasure against such density nonuniformity, the following method has been proposed. The method will be described below with reference to FIGS. 14A to 15C. According to this method, in order to complete a print area shown in FIGS. 12A to 13C, the multi-head 91 is scanned (main scan) three times, and a half area in units of four pixels is completed by two passes. In this case, the eight nozzles of the multi-head are divided into two groups respectively including upper four nozzles and lower four nozzles. Dots to be printed by one nozzle in a single scan are obtained by thinning out given image data to about a half according to a predetermined image data arrangement. In the second scan, dots corresponding to the remaining half image data are recorded, thus completing the area in units of four pixels. The above-mentioned recording method will be referred to as a divisional recording method hereinafter.
When such a recording method is used, even if a head equivalent to the multi-head shown in FIG. 13A is used, since the influence of the nozzles to a printed image is reduced to half, an image shown in FIG. 14B is printed, and black and white lines observed in FIG. 13B do not become conspicuous. Therefore, the density nonuniformity is remarkably eliminated as compared to FIG. 13C, as shown in FIG. 14C.
Upon execution of such recording, image data is divisionally thinned out to predetermined complementary arrangements in the first and second scans. As the image data arrangement (thinning pattern), a checker pattern in which dots are printed on every other pixels in the vertical and horizontal directions is normally used, as shown in FIG. 15A. Therefore, a unit print area (in units of four pixels) is completed by the first scan for printing dots in a checker pattern and the second scan for printing dots in a reverse checker pattern.
FIGS. 15A, 15B, and 15C explain how to complete a predetermined record area using the checker and reverse checker patterns by the multi-head having eight nozzles like in FIGS. 12A to 14C. In the first scan, dots are recorded in a checker pattern using lower four nozzles (FIG. 15A). In the second scan, a sheet is fed by four pixels (1/2 the head length), and dots are recorded in a reverse checker pattern .largecircle. (FIG. 15B). Furthermore, in the third scan, the sheet is fed by four pixels (1/2 the head length), and dots are recorded in the checker pattern again (FIG. 15C). In this manner, when the paper feed operation in units of four pixels, and recording operations of the checker and reverse checker patterns are alternately performed, a record area in units of four pixels is completed for each scan.
As described above, since the print area is completed by two different groups of nozzles, a high-quality image free from the density nonuniformity can be obtained.
Such a recording method has already been disclosed in Japanese Laid-Open Patent Application No. 60-107975 and U.S. Pat. No. 4,967,203, and these references describe that this method is effective to remove the density nonuniformity and connection lines. The former reference discloses that "the invention is characterized by comprising means for forming an overlapping portion by overlapping two adjacent main scans by setting a paper feed width of each main scan to be smaller than the width of the main scan, and means for printing dots of the overlapping portion so as not to overlap each other in the two main scans". According to this reference, as described above, a thinning mask is defined as one for "alternately printing odd and even rows in every other columns" in one case. However, in another case, odd rows are printed in the first scan, and even rows are printed in the second scan. In still another case, odd and even rows are randomly printed in each scan. Thus, the thinning mask and the paper feed width are not completely limited.
In contrast to this, the latter U.S. Pat. No. 4,967,203 discloses that
"a) in the first pass, dots are printed at alternate pixel positions, which are not two-dimensionally adjacent to each other, of only the upper half of a first band, PA1 b) in the second pass, dots are printed on pixel positions, which are not printed in the first pass, in the first band, and at alternate pixel positions, which are not two-dimensionally adjacent to each other, in the lower half of the first band, and PA1 c) in the third pass, dots are printed at pixel positions, which are not printed in the first and second passes, in the first band, and at the same time, the first pass print operation in the next band is performed". In this manner, in this reference, a thinning mask used in divisional recording is limited to an alternate pixel arrangement in which pixels are not two-dimensionally adjacent to each other.
Such divisional recording is also effective for saving the power supply capacity for driving the head. When divisional recording is not performed, all the nozzles must be ready to perform recording in a 1-pixel width in the main scan direction. However, when the above-mentioned divisional recording is performed, the head need only be driven by an energy corresponding to half a total number of nozzles. Therefore, when a recording operation is to be completed in a single driving operation using all the nozzles in the head, the two-divisional recording requires only half a power supply capacity as compared to a case wherein no divisional recording is performed. Furthermore, even when the number of nozzles to be simultaneously driven is limited due to a limited power supply capacity, if a block arrangement in a block driving operation is designed in correspondence with a thinning mask used in divisional recording, the carriage speed can be increased.
However, even when the carriage speed is increased, since the divisional recording requires a plurality of carriage scans per unit area, the divisional recording requires considerable time cost per page, and the throughput is inevitably lowered. In this case, in order to shorten the print time, a method of reciprocally print-scanning a carriage is proposed. According to this method, since all carriage scans each for returning the carriage to the home position without performing any record operation after one record scan can be omitted, the record time per page can be reduced to almost half. In practice, the reciprocal print operation is popularly adopted as a monochrome print method. However, in a color ink jet apparatus having the arrangement of the present invention, the reciprocal print method is not put into practical applications yet for the following reasons.
FIG. 16 is a sectional view of a normally used recording ink and a landing state of the ink printed on a medium (paper sheet). FIG. 16 illustrates a state wherein two different color inks (dots) are absorbed (recorded) at almost neighboring positions to have a time interval therebetween. It is to be noted that, in an overlapping portion of two dots, the subsequently recorded dot tends to extend under the previously recorded dot in the sheet depth direction. Such a phenomenon is caused for the following reason. That is, in a process wherein a dyestuff such as a dye in the ejected ink is physically and chemically coupled to a recording medium, since the coupling capacity between the recording medium and the dyestuff is finite, the previously ejected ink dyestuff is preferentially coupled to the recording medium as along as there is no large coupling force difference depending on the types of dyestuffs, and remains in a large amount near the surface portion of the recording medium. Conversely, the subsequently ejected ink dyestuff is not easily coupled to the surface portion of the recording medium, and is fixed after it sinks deep in the sheet depth direction.
In this case, even when two different inks are printed at a single landing point, a priority color varies depending on the print order of the two different inks, and consequently, two different colors are expressed for visual characteristics of man. For example, assume that four color heads are arranged in the order of black, cyan, magenta, and yellow from the right, and main scans are performed by reciprocally moving the heads in the head alignment direction (right-and-left direction). In a forward scan, the heads are moved rightward, and simultaneously perform recording. At this time, since the recording order on a sheet surface follows the alignment order of the heads, for example, when a green (cyan+yellow) signal is input to a given area, inks are absorbed by each pixel in the order of cyan and yellow. Therefore, as described above, in this scan, the previously absorbed cyan serves as the priority color, and a cyanish green dot is formed. Conversely, in a backward scan after a paper feed operation is performed in the sub-scan direction, the four heads are located at the right side in FIG. 16, and perform recording while being moved in a direction opposite to the forward scan. Therefore, the print order is reversed, and in this scan, a yellowish green dot is formed. When such scans are repeated, cyanish green dots and yellowish green dots are recorded according to the forward and backward movements of the recording heads. If each scan does not use the divisional print method and the paper feed operation is performed by the head width after each of the forward and backward scans, a cyanish green area and a yellowish green area alternately appear by the head width, and a green image which should be a uniform image, is considerably deteriorated.
However, this defect can be slightly conquered using the conventional divisional recording method. In the divisional recording method, as has been described above with reference to FIGS. 15A to 15C, pixels (dots) are recorded half and half by the forward scan for printing cyanish green dots (FIG. 15A or 15C), and the backward scan for printing yellowish green dots (FIG. 15B). Therefore, the color tone of a given area is relaxed by the dots having the two different color tones.
The arrangement and effect of the above method have already been disclosed in U.S. Pat. No. 4,748,453. In this reference, although the paper feed amount is not limited, dots are complementarily recorded at two-dimensionally alternate pixel positions in two (first and second) or more record scans, thereby preventing beading of inks on a medium such as an OHP sheet. In addition, when a color image is formed, the ink landing order for color-mixed pixels is reversed between the first and second scans (reciprocal recording), thereby preventing color banding (color nonuniformity). Since this reference has as its principal object to prevent beading between neighboring pixels, it is characterized in that dots are recorded at two-dimensionally alternate (non-adjacent) pixel positions in a single scan.
Japanese Laid-Open Patent Application No. 58-194541 by the same applicant as the present invention discloses "a recording method wherein a plurality of recording element arrays are arranged parallel to each other, upon execution of a main scan for recording a dot matrix by reciprocally moving the recording element arrays in a direction perpendicular to the recording element arrays, dots fewer than all dots in at least one of rows and columns of the recording dot matrix are intermittently recorded in a forward main scan, and remaining dots in at least one of rows and columns of the recording dot matrix are intermittently recorded in a backward main scan, so that the forward and backward main scans have different overlay recording orders of overlay record dots using the plurality of recording element arrays". In this reference as well, there is no limitation such that the paper feed width is set to be smaller than a normal width unlike in the previously described divisional recording, and the effect of this reference is to prevent deterioration of image quality caused by color mis-registration (color nonuniformity) of a recorded image caused by overlay recording of color inks. Since this reference has as its principal object to prevent color mis-registration, dot positions to be recorded in each scan are not particularly limited. In the embodiments of this reference, a horizontal thinning pattern used for alternately recording dots in only the vertical direction, and a vertical thinning pattern alternately repeated in only the horizontal direction are described in addition to checker patterns (checker and reverse checker patterns).
Also, Japanese Laid-Open Patent Application No. 55-113573 discloses an arrangement for performing reciprocal recording using checker patterns (checker and reverse checker patterns) although it is not limited to a color printer. This reference inhibits continuous print operations of neighboring dots, thereby preventing a dot distortion caused by printing a neighboring dot before a previously printed dot is dried. Therefore, in this reference, a thinning mask is limited to a figured or twilled pattern like in U.S. Pat. No. 4,748,453.
The three references presented above have as their objects to prevent color nonuniformity and beading in reciprocal recording. Therefore, these references do not employ an arrangement in which "the paper feed amount between adjacent scans is set to be equal to or smaller than a normal head width", which arrangement is employed for the purpose of preventing the density nonuniformity caused by variations of nozzles, unlike in the divisional recording method described in this specification.
As described above, when the divisional recording method is adopted in reciprocal recording, since two different groups of record pixels formed in the opposite print orders of color inks can be uniformly arranged in a record area, it is expected that multi-color bi-directional recording, which easily causes color nonuniformity normally, can be realized.
However, even when the above-mentioned divisional recording shown in FIGS. 15A to 15C is performed, the defect of color nonuniformity is not perfectly removed yet. The reason for this will be described below. In general, the amount of an ink droplet is designed so that the ink spreads wider than an area for each pixel on a sheet surface. This is to eliminate any blank portion in an area corresponding to a print duty of 100%. Therefore, even when the divisional recording method is executed, although record pixels themselves are printed at only 50%, an almost 100% area of a recording medium (recording sheet) is covered by dots, as shown in FIG. 17. FIG. 18A is a sectional view of the sheet surface in this case. In FIG. 18A, a checker print operation is performed on a blank sheet in the first pass (forward scan), and a reverse checker print operation is performed in the second pass (backward scan). Reference numeral 2001 indicates a state of inks immediately after the print operation in the first pass (forward scan). In this state, a solid black portion represents a cyan ink, and a hatched portion represents a yellow ink. Since the yellow and cyan inks are printed at an identical position to have a very small time interval therebetween, when they are absorbed by the sheet, the cyan ink is less blurred in a high-density state, and the yellow ink is largely blurred to extend to portions under and around the cyan ink in a low-density state. Also, at this time, the absorbing range of these inks extends over neighboring pixel positions, and as shown in FIG. 17, almost the entire sheet surface is filled with the ink dots.
In the second pass (backward scan) performed under this condition, dots land on the sheet surface on which neighboring ink dots are absorbed, as indicated by reference numeral 2003. Since the second pass is a backward scan, the yellow ink is printed first, and the cyan ink is printed second (2002). When the inks are absorbed in this state, an absorbing state in which both the colors do not clearly appear on the surface is finally formed, as indicated by reference numeral 2003. In a completed image, the density of the cyan ink, which was printed first, is emphasized most strongly, and a green image having cyan as a priority color tone is formed on this print area. Conversely, in a print area which has a backward scan as the first pass, and is adjacent to the above-mentioned print area, the situations of the cyan and yellow inks are reversed, and a green image having yellow as a priority color tone is formed.
FIG. 19 illustrates a state wherein the above-mentioned two print areas appear. More specifically, green images having cyan as a priority color tone are formed on the first and third areas on a recording medium since the cyan ink is printed first in the first and third scans, and green images having yellow as a priority color tone are formed on the second and fourth areas since the yellow ink is printed first in the second and fourth scans. As can be seen from FIG. 19, the lower half nozzles of the heads determine the priority color of each area, and the priority color is reversed between the forward and backward scans. Since two areas having the different priority colors are alternately formed, color nonuniformity still appears in the divisional print method, and deteriorates an image, thus preventing practical applications of the reciprocal print operations.
Furthermore, the defect caused by blurring of an ink to the neighboring pixel position is observed not only as the color nonuniformity but also in monochrome reciprocal print operations. Such a defect will be explained below. FIGS. 20A to 20C show the absorbing state of a monochrome ink in the first and second passes like in FIGS. 18A and 18B. In FIG. 20A, a state 2101 represents a landing state immediately after the first pass print, and states 2102 and 2103 represent landing states after the second pass print when viewed in a sheet section. In the state 2102, the second pass print is performed immediately after the first pass print, and in the state 2103, the second pass print is performed after a certain delay time after the first pass print. These two states cause different absorbing states of the ink recorded in the second pass to the sheet surface. That is, in the state 2102, the ink is absorbed deep in the sheet depth direction, while in the state 2103, the ink printed in the second pass extends on the sheet surface. These states are also confirmed from the rear sheet surface side. That is, the ink in the state 2103 considerably penetrates the sheet to the rear surface side as compared to the state 2102. These states also appear as a density difference on the sheet surface (2104 and 2105).
The time interval generated by reciprocally scanning the carriage is sufficient with respect to the order of the time difference that causes the density difference between the above-mentioned states. This factor appears as a new defect upon execution of the reciprocal print operations. This defect will be described below with reference to FIG. 21.
In FIG. 21, the head performs a forward scan in the direction of an arrow from a position 2201 to perform recording corresponding to a first scan width. After the head performs recording for one line, a sheet is fed by a width 1/2 the scan width, and the head then performs a backward scan in the opposite direction in turn from a position 2202 shown in FIG. 21. Furthermore, after the sheet is fed by the same width as described above, the head performs the forward scan again from a position 2203 to perform recording in the direction of the arrow. Recording intervals of the second pass at positions 1 to 6 of the print area completed at this time are compared. More specifically, at positions 3 and 4, after the first pass print is completed, the second pass print is performed immediately after the sheet is fed by a 1/2 width. In contrast to this, at positions 1 and 6, after the first pass print, the second pass print is performed after an elapse of a time required for reciprocally scanning the carriage once. At positions 2 and 5, the two print operations are performed at just an intermediate time interval. Therefore, as has already been described above with reference to FIGS. 20A to 20C, the positions 1 and 6 have the highest density, the positions 2 and 5 have the next highest density, and the positions 3 and 4 have the lowest surface density since the ink is absorbed deepest. Therefore, the density nonuniformity appears on the left-hand side area where the positions 1 and 4 repetitively appear at an interval of the 1/2 width in the vertical direction, and on the right-hand side area where the positions 3 and 6 repetitively appear at the interval of the 1/2 width in the vertical direction, thus deteriorating image quality. Such a density nonuniformity caused by the carriage reciprocal scanning time, and regularly appearing in the paper feed direction will be referred to as a time interval nonuniformity hereinafter. As described above, the blurring state to non-print pixel positions in the first pass causes dependency of the density on the recording interval between the first and second passes, and it can be understood from this respect as well that actual applications of the reciprocal print method have been impossible so far. In the above description, monochrome recording has been exemplified. This phenomenon also appears together with color nonuniformity in mixed-color recording, as has already been described above, and in this case, it is recognized as right and left different color nonuniformity portions or different color tones.
In one-directional recording as well, the following factor is known as a defect influencing the recording time interval. When the recording apparatus performs a head recovery scan to maintain its own driving scans during recording or waits for transfer of record data, the carriage is temporarily set in a rest state. Such a rest state causes density nonuniformity which occurs irregularly on the order still larger than that of the time interval nonuniformity described above. More specifically, when the carriage is set in a rest state after the first pass print is completed, and the second pass print is performed after some time interval, a corresponding record area has a higher density than other areas. The density nonuniformity caused by such a factor will be referred to as rest nonuniformity to be distinguished from the time interval nonuniformity.
As described above, when the divisional recording or the bi-directional print method is realized to achieve high image quality and high-speed image formation in an ink jet recording apparatus for performing image formation by scanning recording heads in a direction different from the nozzle alignment direction of one head, image defects such as color nonuniformity, rest nonuniformity, and time interval nonuniformity remain unremoved.